1. Field of the Invention
The present invention relates generally to a fuel injection control method for a cylinder injection type internal combustion engine for a motor vehicle in which fuel is injected directly into engine cylinders, and a system for carrying out the method.
2. Description of Related Art
As the fuel injection control system for the cylinder injection type engine which is designed for injecting the fuel directly into the cylinders of a gasoline engine, there may be mentioned a control system disclosed, for example, in Japanese Unexamined Patent Application Publication No. 237854/1992 (JP-A-4-237854). The cylinder injection type internal combustion engine attracts attention as an ideal engine promising advantageous and profitable effects which may be globally classified into four types, as mentioned below.
1. Reduction of Harmful Gases Contained in Engine Exhaust Gas
In the conventional fuel injection system in which the fuel is injected by the injector mounted externally of the engine cylinders, a part of the fuel injected from the injector will adhere to inner walls of the intake pipe, surfaces of the intake valves of the engine and others. Such fuel deposition has to be taken into consideration particularly when the engine is operating at a low temperature such as engine starting operation, which is unfavorable for vaporization of the fuel or when the engine is in a transient operation state where the amount or quantity of fuel to be supplied to the engine has to be changed at a relatively high speed. By contrast, in the cylinder injection type internal combustion engine, the air-fuel mixture can be made lean without taking into consideration the delay involved in the transportation of the fuel, whereby the amounts of harmful HC (hydro carbon) and CO (carbon monoxide) contained in the exhaust gas can be reduced significantly.
2. Reduction of Fuel Cost
Because the fuel is injected immediately before the ignition timing upon injection of the fuel into the engine cylinders, there is formed a mass of combustible fuel mixture around the spark plug at the time point for ignition, making nonuniform the distribution of the gas mixture containing the fuel within the cylinder. Thus, the fuel-air mixture undergoes a so-called stratified combustion. Consequently, the air-fuel ratio in appearance between the amount of air and that of the fuel charged into the cylinder can be significantly decreased so that the air-fuel mixture is rendered lean correspondingly. Besides, owing to the stratified combustion, combustion of the air-fuel mixture is scarcely subjected to adverse influence of the exhaust gas recirculation with increased ratio. By virtue of these features in addition to lowering of the pumping loss, fuel-cost performance of the engine system can be enhanced.
3. Increased Output Power
Since the combustion of the air-fuel mixture occurs concentratedly around the spark plug owing to the stratified combustion, the amount of end gas (i.e., air-fuel mixture gas existing in regions remote from the spark plug) decreases, whereby the anti-knocking performance of the engine can be enhanced with the compression ratio being increased. Furthermore, because the fuel is converted into gas (i.e., gasified) within the cylinder, the intake air is deprived of heat as vaporization heat. Consequently, the density of the intake air increases, which is effective for increasing the volumetric efficiency and hence the output power of the engine.
4. Improvement of Drivability
By virtue of the direct fuel injection into the cylinder, the time taken for generating the output torque by the engine from the fuel injection through intervention of the fuel combustion is shorter when compared with the conventional engine in which the fuel is injected externally of the engine cylinder. Thus, the engine system capable of responding to the demands of a driver with high speed can be realized.
Although the cylinder injection type internal combustion engine has advantageous features such as mentioned above, it suffers some problems. Namely, because the fuel (gasoline) is directly charged into the cylinder, the air-fuel mixture tends to converge around the spark plug when compared with the conventional engine in which the fuel is charged by the injector mounted externally of the engine cylinder, as a result of which the spark plug is susceptible to deposition of soot. In particular, in the cylinder injection type engine in which the fuel injection is performed in the explosion stroke, the soot deposition phenomenon is more likely to take place because of intensive concentration of the fuel around the spark plug. In this conjunction, it is noted that when the engine operation is stopped in the state where soot is deposited on the spark plug, the starting performance of the engine in the succeeding engine starting operation will be accompanied with degradation because of the soot deposition on the spark plug. In particular, when the fuel pump designed to operate in synchronism with the engine rotation is employed, the fuel injection pressure is likely to vary because the engine rotation speed is intrinsically low in the engine stating operation, incurring thus instability of the fuel supply to the cylinder. Furthermore, when the engine temperature is low, an increased quantity of fuel has to be injected, which results in that the unwanted phenomena mentioned above make a more marked appearance, to further disadvantage.
In the cylinder injection type engine known heretofore, when the ignition switch is turned off by the driver, the power supply to the ignition system is interrupted by an external circuit, making it impossible to perform ignition control. Consequently, the engine operation will be stopped in the state where the fuel as injected remains unburned when the ignition switch is opened, which is unfavorable for the succeeding starting operation of the engine.